454
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion in certain cases is due solely to phase transition. There is some indication that crystallization contributes to this effect, but further studies of flow curves and investigations of the extended samples with x-rays, by swelling experiments, or by the new method of Nickerson (28) will be necessary to clear the situation. As to the second-order phase transition of high polymers, it seems that the freezing in of the internal Brownian movement is connected with the brittle point of the material. Particularly, polyhydrocarbons and polyesters show a distinct tendency to pass from a rubberlike state at higher temperatures to a brittle (glassy) state as soon as a certain rather sharp temperature limit is reached. Bekkedahl and Xood (2, 53) determined this transition point for rubber to be -80" C.; others (3, 18, 19) found that the brittle point of polystyrene is around $80" C. The paper of Carswell, Hayes, and Nason (page 454) gives additional convincing experimental evidence of this point. Summarizing, our present experimental knowledge suggests a distinct influence of both types of phase transitions on the mechanical behavior of high polymers, although it is not yet possible to connect quantitatively the amount of converted material with the magnitude of the mechanical effect.
Literature Cited (1) rllfrey, T., Rubber Chem. Tech., 14, 525 (1941). (2) . . Bekkedahl, N., J . Research Natl. Bur. Standards, 13, 411 (1934); 23, 571 (1939). (3) Bennewitz, K., and Rotger, H., P h y s i k Z.,40, 416 (1939). (4) Bingham, E. C., "Fluidity and Plasticity", p. 217 (1922).
Vol. 34, No. 4
Brillouin, M., Am. chim. phys., [ 7 ] 14, 311 (1898). Burgers, J. M., "First Report on Viscosity and Plasticity", 2nd ed., pp. 5 and 73 (1939). Clark, G. L., IYD.ENG.CHEX.,31, 1379 (1939). Davies, J. M.,Miller, R. F., and Busse, 1%'. F., J . Am. Chem. Soc., 63, 361 (1941). Eyring, H., Powell, R. E., and Roseveare, W. E., IKD. ENG. CHEM.,33,430 (1941); J . Am. Chem. Soc., 62, 3113 (1940). Field, J. E., J . Applied P h y s . , 12, 23 (1941). Fuller, C. S., Chem. Reu., 26, 143 (1940). Fuoss, R. M., and Kirkwood, J. G., J . Am. C'hem. Soc., 63, 385 (1941). Gehman, S. D., and Field, J. E., J . Applied Phys., 10, 564 (1939). Gemant, A,, Naturwzssenschafteia, 23, 406 (1935). Guth, E., and James, H. M., IND. EXQ.CHEX.,33, 624 (1941). Hermans, P. J., and Kratky, O., Kolloid-Z., 86, 246; 89, 345, 349 (1939). Hohenemser, K., and Frazer, UT.,J . Rheol., 3, 16 (1932). Holzmiiller, W.,and Jenckel, E., 2. physik. Chem., A186, 359 (1940). Houwink, R., "Elasticity, Plasticity and Structure of Matter", 1-II.. Q27
Jeffreys, H., "The Earth", 2nd ed., p. 263 (1929). Kuhn, Vi'., Kolloid-Z., 68, 2 (1934). Leaderman, H., Testile Research, 11, 171 (1941). Mack. E . . J . Phus. Chem.. 41.221 11937). Maxwell, J. C., k h i l . Mag., [4],35, 134 (1868). Meyer, K. H., NaturzLissenschaften, 16, 781 (1928). Meyer, K. H., and Lotmar, W., Helv. Clcim. Acta, 19, 68 (1936). Morss, H. 9., J . Am Chem. Soc., 60, 237 (1938). Nickerson, R. P., IND.ENG.CHEM.,33, 1022 (1941). Poole, H. J., Trans. Faraday SOC.,21, 114 (1925). Vogt, W., 2. physrk. Chem., 47, 671 (1892). Warren, E. B., and Simard, G. L., J . Am. Chem. Soc., 58, 507 (1936). Wohlisch, E., Kolloid-Z., 89, 239 (1939). Wood, L. A., Proc. Rubber Tech. Conf., London, 1938, 933.
Physical Properties of Polystyrene as Influenced by Temperature Preparation of Test Specimens and Methods of Testing
T. S. CARSWELL, R. F. HAYES, AND H.K. NASON Monsanto Chemical Company, Springfield,
Mass.
HE physical properties of plastics are markedly inT fluenced by ambient temperature, but comparatively few quantitative data on this subject have been published for polystyrene. The variation of flexural strength and deflection a t break for compression-molded polystyrene and for a number of other compression-molded plastic materials at temperatures from -70" to f200" C, (-94' to +392" F.) was described by Nitsche and Salewski (18). Jenckel and Lagally (6) determined the tensile strength of extruded polystyrene filaments at 30" t o 60" C. (86" to 140" F.). The elongation a t 20" to 90" C. (68" to 194" F.) was reported for extruded and racked polystyrene foil by Muller (11). Since t h e mechanical properties of plastic materials are profoundly influenced by the methods used in preparing the test specimens, data on such properties are meaningless unless the details of preparation are also known. This paper describes variations in some of the mechanical properties of injection-molded polystyrene over the range froin -75" to +loo" C. (-103' to +212" F.). This method was chosen because injection molding is by far the most commonly used commercial process for the fabrication of polystyrene.
Test specimens were prepared from three grades of polystyrene whose average molecular weights were, respectively, 60,000, 95,000, and 115,000. Molecular weights were calculated from the viscosities of 0.2 per cent solutions of the polymers in toluene by means of the Staudinger relation (18):
where T~~ = specific viscosity of solution c = concentration of solution, unit moles of polymer 1
M ICm
liter
= =
molecular weight the constant 1.8 X lo-'
The specimens were injection-molded on a Reed-Prentice injection-molding machine of 2-ounce capacity. The test specimen for the tensile and flexural tests mas of the dumbbell type (Figure 1) with a 4-inch length of uniform l / k X '/4 inch cross section. These specimens were injection-molded at a heater temperature of 440' F., a ram pressure of 1100-1250 pounds per square inch, and a total cycle of 50 seconds. The specimen for the impact and hardness tests was a 5-inch X l/2 inch cross section. Injection-molding conlength of ditions were: heater temperature 450" F. ; ram pressure, 1200 pounds per square inch; total cycle, 60 seconds. A11
April, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
455
(or range) in the elongation and relaxation time curves for polystyrene reported by Muller ( I I ) , and to Russell's brittle point (16). I n this respect polystyrene has a behavior similar to that of rubber, whose volume-temperature curve ( 3 ) exhibits a second-order transition range a t -71 " to -67" C. (-95.8" to
Figure I. Monsanto Tensile Test Specimen Dimensions in inches
specimens were allowed to stand at least 72 hours a t 25' C. (77" F.) and 50 =t2 per cent relative humidity before starting tests. Tensile and flexural data were obtained on a screw type tester with a constant rate of cross-head movement. A n insulated box (Figure 2) was constructed to house the grips for the tensile test and the supports for the flexural test. Testing temperature, which was varied from -25" to +loo" C. (-13" to +212" F.) was obtained by circulating through the insulated box dry air t h a t had been heated by contact with a hot tube or cooled by contact with solid carbon dioxide. Temperature control accurate to within * 1.0"for the range 0" to 90" C. and to within h 2 . 0 " at -25" and a t +loo" C. was obtained by this method. No attempt was made to control humidity since the moisture absorption of polystyrene is negligible. Both tensile and flexural data were obtained using a constant rate of jaw separation of 1inch per minute. Distance between supports in the flexural test was 2 inches. The '/s X '/2 inch bars for the impact test were conditioned for several hours a t the desired temperature and then tested immediately after removal from the conditioning atmosphere. Both Izod and Charpy test methods were used (1). The l/z X l/z inch bars remaining from the impact test were used for the indentation hardness test. These specimens were conditioned for several hours at the desired temperature, and were then tested as rapidly as possible after removal from the conditioning chamber. The Rockwell apparatus was used (a), and all determinations were made on the M scale ('/r-inch ball, 100-kg. major load).
The tensile, flexural, impact, and hardness properties of injection-molded polystyrenes of several molecular weight ranges have been investigated at temperatures from - 75' to +I 00" C. Injection-molded polystyrene undergoes a critical transformation with respect to mechanical properties at 80" to 85' C. which is analogous to the "second-order transition range" of rubber at 71 O to -67" C. Polystyrene becomes tougher as the ambient temperature is reduced below the transformation renge, and differs in this respect from most cellulose ester or ether plastics which generally are more brittle at lower temperatures. Tensile strength, elongation, and impact strength of injection-molded polystyrene increase as the average molecular weight of the polymer increases. Indentation hardness appears to be independent of molecular weight at temperatures below the transformation range.
-
Tensile and Flexural Properties Tensile and flexural data a t various temperatures from -25" to f100" C. are presented in Table I and plotted in Figure 3. Both tensile strength and elongation increase with increase in average molecular weight of the polymer. The relation between flexural properties and molecular weight is not so clearly defined by these tests. A critical transformation occurs a t 80" to 85" C. (176" to 185" F.) for all of the materials tested. At this temperature polystyrene changes from a hard, glasslike solid to a soft plastic. This transformation temperature coincides with the inflection point in the density-temperature curves for polystyrene reported by Patnode and Scheiber (IS) and by Ueberreiter et al. (6, 7, 17). It also corresponds to the inflection point
Figure 2.
Apparatus Arranged for Tensile Tests
456
INDUSTRIAL A N D ENGINEERING CHEMISTRY 1qooo
I
I
I
,
TABLEI. TENSILEAND FLEXURAL DAT.4 Temp. of Testing,
C. (" F.) 25-(
- 13)
(32) 25 (77)
BO (140)
-20
%O
0
20
40
'
60
80
100
80 (176)
Cn
I I
85 (1863 90 (194) 100 (212)
V+T 60,000 e.oc Av. MOL. -20
-LO
;j: eqooo
h 3
r
0
e0
x
Av. MOLVICI: 115,000 Av MOL.?&?. 95,000
A
Av. MoLWT. 60,000.
Ultimate Tensile Av. Mol. Strength, Weight Lb./Sq. In. 60,000 7220 95,000 8130 115,000 8540 60,000 6300 95,000 7230 115,000 7700 60.000 5330 95,000 6190 115,000 6550 60,000 4350 95.000 4860 115,000 5030 60,000 3840 95,000 3650 115,000 4260 60,000 95,000 115.000 60,000 1510 95,000 1710 116,000 2240 60.000 305 96,000 199 115,000 302
.. .. ..
Ultimate Elongation
%'
2.8 3.3 3.2 2.4 3.0 2.9 2.2 2.2 2.7 2.0 2.1 2.4 2.1 2.0 2.4
..* .
O S POLYSTYRENE'
Ultimate Flexural Strength Lb./Sq. Iz. 20,900 21,600 22,200 19,350 20,500 18,400 15,550 16,000 16,500 14,800 10,100 12,450 8,000 8,160 9,560
..
33 63 28 147 140 152
7,*i00 10,580
.... ..
.... ..
Ultimate Deflection, Inches 0.133 0.149 0.149 0.135 0.155 0.128
0.103 0.126 0,112 0,082 0,095 0.097 0.088 0.076 0.103 No breakb No breakb 0.090 .,..
....
.... .... .... .
I
.
.
The average values for mean deviation of test results were: tensile strength, +=ZOO lb./sq. in. : elongation, + O . l % : tflexural strength, t 7 0 0 Ib./sq in: deflection, *0.007 inch. b Withih deflection capacity of testing machine. 0
40
60
I 60
100
1
~ 0 8 - 0
Vol. 34, No. 4
A
-88.6" F.). I n rubber, too, this range is characterized by marked changes in mechanical properties and in heat capacity (3,4,8,9,iC, 1 5 , 1 9 , 2 2 ) . Ueberreiter and his eo-workers (e,?, 17) pointed out this similarity in the shape of volume-temperature curves. At temperatures above its transition range, polystyrene displays elastic properties and thermal retraction as pointed out by Whitby (20,81),and by Meyer, Susich, and Valk6 (io), and in these respects also the similarity to rubber is striking. I n fact, polystyrene may be considered to be a rubberlike polymer whose critical transformation range occurs a t a somewhat higher temperature than that of natural rubber. As the ambient temperature is reduced below 80' to 85" C., the tensile and flexural strengths of polystyrene increase. This is the general tendency of plastic materials. However, the elongation and deflection a t break of injection-molded polystyrene also increase as the temperature is reduced, and this trend continues down to a t least -25" C. This effect is unusual in a thermoplastic, as most such materials show reduced elongation and reduced ultimate deflection as the temperature is lowered. [Jenckel and Lagally (5) and Muller (11) reported elongation to decrease as the temperature was reduced. However, these workers studied extruded filaments or foils, both of which were highly oriented by racking, and this may be expected to influence properties.] These observations indicate that injection-molded polystyrene does not become more brittle as the temperature is reduced but, on the contrary, actually becomes tougher under these conditions. I n this respect it differs from most other plastic materials, especially from plasticized cellulose esters or ethers, most of which show greatly decreased toughness (i. e., increased brittleness) a t the lower temperatures.
Impact Strength
Figure 3. Tensile and Flexural Properties of Injection-Molded Polystyrene
Impact streilgth data were obtained a t a series of teniperatures from - i 5 " t o +io" C. (-103' to 167' F.). These data (Table 11) confirm, in general, the results of the tensile and flexural tests, and sho.ic.that there is at least no decrease in the toughness of injection-molded polystyrene as the tem-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
April, 1942 .
PO,
457
I
Av. Ma.W. 115,000 \
0
-
100
d
2
\
d
1
76
z
AV MOL. W. 60,000f
j
50
1:g as
i:
o!
1.
3-16
perature is reduced below the transformation range and that this trend continues down to a t least -75" C. The results for the unnotched Izod test are shown graphically in Figure 4. (In general, the unnotched Izod method has been found the most satisfactory for testing the impact strength of polystyrene.) The general increase in toughness with increase in average molecular weight of the polymer, which is indicated by the tensile test results, is confirmed by these impact data.
-
o Av Moc WT II5,OOO x Av MOL W T 60,000 60
-75
-80
-40
0
40
TLMPCRATURS,
Fi
BO
c.
ure 5. Indentation Hardness o Injection-Molded Polystyrene
9
Indentation Hardness Hardness data, on the Rockwell M scale, are shown in Table 111and Figure 5. Here again, a rapid change in values a t or near the transformation temperature is noted. Also,
obtained by conditioning the test specimens t o thermal equilibrium a t the des i r e d temperature and then testing as rapidly as possible on the Rockwell apparatus which was a t a temperature of 25" C. Hence, t h e hardness values given for -70" and -25" C. are probably slightly lower than the true values for these temperatures, and the values for 50°, 70°, 80°, and 90" C. are probably slightly high.
Practical Importance This increase of toughness at low temperatures combined
with other favorable physical properties (e. g., negligible water absorption, good dimensional stability, freedom from indentation hardness is practically independent of molecular taste and odor, wide range of color, and excellent moldability) weight at temperatures below the transformation range. has been an important factor in the growing application of polystyrene in those industrial fields where-physical and mechanical stability are important. Panels, drip trays, doors, TABLE11. IMPACT DATA etc., in recent models of mechanical reImpact Strength, In.-Lb. per 0 . 5 X 0.5 In. Bar frigerators are good examples of such Av. Mol. -75' C. -30' C. 260 c. 70" C. applications. Test Method Weight ( - 1 1 2 O F.) ( - 22' F.) (77' F.) (158' F.) 23.8 * 1.8 21.7 * 2.1 17.6 0.8 Charpy, unnotched 60.000 24.2 1.5 Acknowledgment f
Izod, unnotched Charpy, notched Izod, notched a
115,000 60.000 115,000 60,000 115000 60:OOO 115,000
A1.2 15.0*1.4
25.7
18.0 * 1.5 1.82 * 0.03 1.80 * 0.03 2.2AO.3 1.810.5
f
26.3 * 4 . 2 14.7if0.7 16.6 * 1.1 1.80 if 0.03 1.77 f 0.03 1.710.5 2.1*0.3
14.7*0.6 18.3 * 1.3 1.89 * 0.04
24.5 r 2 . 4 12.2*0.9 14.6 * 1.8 1.80 * 0.06
2.320.1 1.7t0.3
1.920.4 1.9if0.25
26.2 1 2 . 0
Clean, straight breaks were not obtained, and the values were very irregular.
TABLE111. INDENTATION HARDNESS DATA Conditioning Tern C. fd'F.)
Hardness a t Av. Mol. Wt. of: 60,000 124 102 94 82 64 48 29 70
-
115,000 124 102 92 82 63 40 31 20
-
The exact determination of hardness was complicated by the impracticability of exposing the testing apparatus to temperatures far above or far below room temperature, and by the rapid change in surface temperatures of cooled or heated specimens in contact with the highly conductive metal parts of the apparatus. The values shown in Table 111 for temperatures of 5" and 25" C. are quite precise, as both specimens and testing machine were a t the indicated temperature when the determinations were made. The other values given are approximate, and were
We are deeply indebted to W. E. Tucker, Jr., who conducted most of the laboratory tests.
Literature Cited (1) Am. SOC.Testing Materials, Standards. Part 111, pp. 252-4, Designation D256-38 (1939). bid., P a r t I, pp. 743-9, Designation E18-36 (1939). edahl, J . Research, Natl. Bur. Standards, 13, 411 (1934). ity, Plasticity and the Structure of Matter", p. 182 (1940). (5) Jenckel and Lagally, 2. Electrochem., 46, 186-8 (1940). (6) Jenokel and Ueberreiter, 2. physik. Chem., A182, 361-83 (1938). (7) Jenckel, Ueberreiter, and Woltman, A t t i X o congr. intern. chim., 4, 184 (1939). (8) LeBlanc and Krager, KoEZoid-Z., 37,205 (1925). (9) Mark and Valk6, Rbv. gin. caoutchouc, 7 (64), 21 (1930). (10) Meyer, Susich, and ValkB, Kolloid-Z., 59, 208-16 (1932). (11) Muller, Wiss. Verbffent. Siemens-Werken, 19, 110-33 (1940). (12) Nitsche and Salewski, Kunststoffe, 29, 209-20 (1939). (13) Patnode and Scheiber, J . Am. Chem. SOC.,61,3449-51 (1939). (14) Rosbaud and Sohmid, 2.tech. P h y s i k , 9, 98 (1928). (15) Ruhemann and Simon, 2. physik. Chem., A138, 1 (1928). (16) Russell, IND. ENCI. CHEM.,32, 509-12 (1940). (17) Ueberreiter, Kunststoffe, 30, 170-2 (1940). (18) Staudinger, "Die hochmolekularen organischen Verbindungen", pp. 52-72 (1932). (19) Whitby, in Davis and Blake's "Chemistry and Technology of Rubber", Chapt. 11, pp. 62-4, 111-12 (1937). (20) Whitby, J . Pltys. Chem., 36, 204-14 (1932). (21) Whitby, T r a n s . Inst. Rubber I n d . , 6, 46-58 (1930). (22) Whitby and Katz, IND. ENG.CHEM.,25, 1346 (1933).
